Firewalls mean nothing to a squirrel or a rat - they get through easily. And when you say "checking the trap" I assume you mean poking your head up there to see what's going on? No, that shouldn't bother them at all, most squirrels are used to human activity.

Leave the light on for them - and you - they are "sight" animals and lights will tend to attract them to the trap if you put it near the light. Also, a trapped squirrel will tend to move around inside the trap and making the treadle move and making noises, often you can hear them without even going up there. Even so, it's best to check it PERSONALLY every few days. Some squirrels will stay quiet when trapped and you'll think there's nothing up there, they won't last longer than 3-5 days without water.

You can also use one of those baby monitors, even the video ones are fairly cheap nowadays. You still should check the trap personally every 3-5 days.

Of course, it might not be REGULAR squirrels, it could be flying squirrels, it depends on where you are - in northern states the flying variety should be hibernating.